7.2.1.1 Water vapour feedback

Water vapour feedback continues to be the most consistently important feedback
accounting for the large warming predicted by general circulation models in
response to a doubling of CO2. Water vapour feedback acting alone approximately
doubles the warming from what it would be for fixed water vapour (Cess et al.,
1990; Hall and Manabe, 1999; Schneider et al., 1999; Held and Soden, 2000).
Furthermore, water vapour feedback acts to amplify other feedbacks in models,
such as cloud feedback and ice albedo feedback. If cloud feedback is strongly
positive, the water vapour feedback can lead to 3.5 times as much warming as
would be the case if water vapour concentration were held fixed (Hall and Manabe,
1999).

As noted by Held and Soden (2000), the relative sensitivity of OLR to water
vapour changes at various locations depends on how one perturbs the water vapour
profile; the appropriate choice depends entirely on the nature of the water
vapour perturbation anticipated in a changing climate. The sensitivity is also
affected by cloud radiative effects, which tend to mask the influence of sub-cloud
water vapour on OLR. Incorporating cloud radiative effects and a fixed relative
humidity perturbation (argued to be most appropriate to diagnosing GCM water
vapour feedback), Held and Soden suggest that OLR is almost uniformly sensitive
to water vapour perturbations throughout the tropics. Roughly 55% of the total
is due to the free troposphere in the "tropics" (30°N to 30°S)
with 35% from the extra-tropics. Allowing for polar amplification of warming
increases the proportion of water vapour feedback attributable to the extra-tropics.
Of the tropical contribution, about two thirds, or 35% of the global total,
is due to the upper half of the troposphere, from 100 to 500 mb. The boundary
layer itself accounts for only 10% of the water vapour feedback globally. Simulations
incorporating cloud radiative effects in a doubled CO2 experiment (Schneider
et al., 1999) and a clear-sky analysis based on 15 years of global data (Allan
et al., 1999) yield maximum sensitivity to water vapour fluctuations in the
400 to 700 mb layer (see also Le Treut et al., 1994). In a simulation analysed
by Schneider et al. (1999) extra-tropical water vapour feedback affected warming
50% more than did tropical feedback.

Most of the free troposphere is highly undersaturated with respect to water,
so that local water-holding capacity is not the limiting factor determining
atmospheric water vapour. Within the constraints imposed by Clausius-Clapeyron
alone there is ample scope for water vapour feedbacks either stronger or weaker
than those implied by constant relative humidity, especially in connection with
changes in the area of the moist tropical convective region (Pierrehumbert,
1999). It has been estimated that, without changes in the relative area of convective
and dry regions, a shift of water vapour to lower levels in the dry regions
could, at the extreme, lead to a halving of the currently estimated water vapour
feedback, but could not actually cause it to become a negative, stabilising
feedback (Harvey, 2000).

Attempts to directly confirm the water vapour feedback by correlating spatial
surface fluctuations with spatial OLR fluctuations were carried out by Raval
and Ramanathan (1989). Their results are difficult to interpret, as they involve
the effects of circulation changes as well as direct thermodynamic control (Bony
et al., 1995). Inamdar and Ramanathan (1998) showed that a positive correlation
between water vapour, greenhouse effect and SST holds for the entire tropics
at seasonal time-scales. This is consistent with a positive water vapour feedback,
but it still cannot be taken as a direct test of the feedback as the circulation
fluctuates in a different way over the seasonal cycle than it does in response
to doubling of CO2.